An isolated switching power converter such as a flyback switching power converter is typically provided with a mobile device for battery charging as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching may be controlled by a primary-side controller that in turn needs to coordinate a cycling of a synchronous rectifier transistor on the secondary side. The primary-side controller cannot directly control the synchronous rectifier transistor through a wire or lead because the ground isolation is then broken. An analogous problem occurs for a secondary-side controller that must control the power switch transistor.
To accommodate the flow of control signals either from the primary side to the secondary side or from the secondary side to the primary side, it is conventional to use optoisolators. But the use of optoisolators is complicated by their wide variation in current transfer ratio and other operating parameters. An alternative is the use of digital isolators to transfer the control signals with high voltage isolation and accurate timing. A digital isolator uses a high-frequency carrier signal to modulate a gate driver signal at the transmitting side. For example, the carrier signal may be one GHz or higher in frequency, having a pulse width distortion of less than 10 ns and a common-mode transient immunity of 50V/ns.
To demodulate the transmitted signal on the receiving side, a digital isolator typically requires a low-pass filter and a relatively-fast comparator. Such digital isolators are expensive and over-qualified for typical flyback converter applications in which the common-mode transient immunity is normally less then 1 v/ns and the pulse-width distortion is larger than 50 ns.
Accordingly, there is a need in the art for flyback converters with improved communication of control signals between the primary and second sides of the transformer.